1. Technical Field
The present invention relates to an enzymatic process for preparing and modifying polymers with pendant carboxylic acid groups. More particularly, the present invention relates to a process for modifying the carboxylic acid pendant groups of polymers with hydroxyl groups of alcohols (polyols) through an enzyme-catalyzed esterification with high selectivity.
2. Prior Art
Methods for the preparation and tailoring of polymers through the esterification of pendant groups on the polymers are known in the art. Conventional methods for the direct esterification of acid groups attached as side or pendant groups to polymer chains with a molecule containing an alcohol group suffer from many problems. High reaction temperatures (>200° C.) and the need for acid catalysts for such reactions result in undesirable side reactions such as dehydration of the alcohol, β-scission of ester links to form acid and alkenes, decarboxylation of the acid, and polymerization of alkenes formed. Also, these harsh conditions make it impossible to conduct reactions with groups that are thermally sensitive or are decomposed in the presence of the acid catalyst. For example, the direct esterification of hydrolytically labile main chains bearing carboxylic acid pendant groups would result in deleterious reactions such as extensive hydrolysis of the main chain. An undesirable alternative is the use of stoichiometric quantities of expensive coupling agents such as those containing carbodiimide moieties.
The prior art discloses a number of examples of methods for the introduction of ester bonds onto pendant groups of polymers. However, most of the prior art is related to the preformation of an ester prior to polymerization and not to the enzyme-catalyzed esterification of carboxylic pendant groups of an existing polymer.
U.S. Pat. No. 2,680,735 to Fegley discloses a method for the formation of a monomeric ester bond on a pendant group that requires a tedious protection-deprotection step. The necessity for the use of inhibitors to protect the monomer from premature polymerization and otherwise harsh chemical methods is a relative disadvantage.
U.S. Pat. No. 2,877,215 to Fang discloses a phoshated copolymer in which an ester bond is created on a pendant group through steps also involving a protection-deprotection step and further involving the use of phosphoric acid. U.S. Pat. No. 2,967,173 to Fang and U.S. Pat. No. 2,979,514 to O'Brien disclose the preparation of polymerizable polyhydroxy esters of acrylic acid and methacrylic acid to homopolymers and copolymers with high precaution using phosgene or a lower alkyl ester of chromic acid in the presence of a hydrogen chloride acceptor. U.S. Pat. No. 3,530,167 to Dowbenko also discloses certain acetal-type polymers, and further requires multiple steps for the synthesis using various hazardous chemicals.
U.S. Pat. No. 4,076,727 to Zey discloses preparing cyclic acetals from polyols, but again uses protection-deprotection steps. U.S. Pat. No. 4,634,722 to Gallop discloses copolymers that also are produced using a protecting group, such as an alkyl boronic acid adduct of glyceryl methacrylate or glyceryl acrylate, in combination with an alkyl acrylate or alkyl methacrylate.
Summarily, the prior art methods rely on performing reactions under relatively harsh chemical conditions and/or the use of protection-deprotection methods to prevent side chains from becoming chemically altered. Accordingly, new and more efficient methods for preparing and tailoring polymers with carboxylic acid side groups are constantly being sought.